feat: TX proposal procedures attribute

This adds support for decoding and retrieving governance proposal
procedures from a TX

Fixes #344

ledger/conway.go

@@ -121,10 +121,10 @@ func (h *ConwayBlockHeader) Era() Era {
 
 type ConwayTransactionBody struct {
 	BabbageTransactionBody
-	TxVotingProcedures   VotingProcedures `cbor:"19,keyasint,omitempty"`
-	ProposalProcedures   *cbor.Value      `cbor:"20,keyasint,omitempty"`
-	CurrentTreasuryValue int64            `cbor:"21,keyasint,omitempty"`
-	Donation             uint64           `cbor:"22,keyasint,omitempty"`
+	TxVotingProcedures   VotingProcedures    `cbor:"19,keyasint,omitempty"`
+	TxProposalProcedures []ProposalProcedure `cbor:"20,keyasint,omitempty"`
+	CurrentTreasuryValue int64               `cbor:"21,keyasint,omitempty"`
+	Donation             uint64              `cbor:"22,keyasint,omitempty"`
 }
 
 func (b *ConwayTransactionBody) UnmarshalCBOR(cborData []byte) error {
@@ -176,6 +176,150 @@ type GovActionId struct {
 	GovActionIdx  uint32
 }
 
+type ProposalProcedure struct {
+	cbor.StructAsArray
+	Deposit       uint64
+	RewardAccount Address
+	GovAction     GovActionWrapper
+	Anchor        GovAnchor
+}
+
+const (
+	GovActionTypeParameterChange    = 0
+	GovActionTypeHardForkInitiation = 1
+	GovActionTypeTreasuryWithdrawal = 2
+	GovActionTypeNoConfidence       = 3
+	GovActionTypeUpdateCommittee    = 4
+	GovActionTypeNewConstitution    = 5
+	GovActionTypeInfo               = 6
+)
+
+type GovActionWrapper struct {
+	Type   uint
+	Action GovAction
+}
+
+func (g *GovActionWrapper) UnmarshalCBOR(data []byte) error {
+	// Determine action type
+	actionType, err := cbor.DecodeIdFromList(data)
+	if err != nil {
+		return err
+	}
+	var tmpAction GovAction
+	switch actionType {
+	case GovActionTypeParameterChange:
+		tmpAction = &ParameterChangeGovAction{}
+	case GovActionTypeHardForkInitiation:
+		tmpAction = &HardForkInitiationGovAction{}
+	case GovActionTypeTreasuryWithdrawal:
+		tmpAction = &TreasuryWithdrawalGovAction{}
+	case GovActionTypeNoConfidence:
+		tmpAction = &NoConfidenceGovAction{}
+	case GovActionTypeUpdateCommittee:
+		tmpAction = &UpdateCommitteeGovAction{}
+	case GovActionTypeNewConstitution:
+		tmpAction = &NewConstitutionGovAction{}
+	case GovActionTypeInfo:
+		tmpAction = &InfoGovAction{}
+	default:
+		return fmt.Errorf("unknown governance action type: %d", actionType)
+	}
+	// Decode cert
+	if _, err := cbor.Decode(data, tmpAction); err != nil {
+		return err
+	}
+	g.Type = uint(actionType)
+	g.Action = tmpAction
+	return nil
+}
+
+func (g *GovActionWrapper) MarshalCBOR() ([]byte, error) {
+	return cbor.Encode(g.Action)
+}
+
+type GovAction interface {
+	isGovAction()
+	Cbor() []byte
+	Utxorpc() *utxorpc.Certificate
+}
+
+/*
+proposal_procedures = nonempty_oset<proposal_procedure>
+
+gov_action =
+  [ parameter_change_action
+  // hard_fork_initiation_action
+  // treasury_withdrawals_action
+  // no_confidence
+  // update_committee
+  // new_constitution
+  // info_action
+  ]
+
+policy_hash = scripthash
+
+parameter_change_action = (0, gov_action_id / null, protocol_param_update, policy_hash / null)
+
+hard_fork_initiation_action = (1, gov_action_id / null, protocol_version)
+
+treasury_withdrawals_action = (2, { reward_account => coin }, policy_hash / null)
+
+no_confidence = (3, gov_action_id / null)
+
+update_committee = (4, gov_action_id / null, set<committee_cold_credential>, { committee_cold_credential => epoch }, unit_interval)
+
+new_constitution = (5, gov_action_id / null, constitution)
+
+constitution =
+  [ anchor
+  , scripthash / null
+  ]
+
+info_action = 6
+*/
+
+type ParameterChangeGovAction struct {
+	// TODO
+}
+
+func (a ParameterChangeGovAction) isGovAction() {}
+
+type HardForkInitiationGovAction struct {
+	// TODO
+}
+
+func (a HardForkInitiationGovAction) isGovAction() {}
+
+type TreasuryWithdrawalGovAction struct {
+	// TODO
+}
+
+func (a TreasuryWithdrawalGovAction) isGovAction() {}
+
+type NoConfidenceGovAction struct {
+	// TODO
+}
+
+func (a NoConfidenceGovAction) isGovAction() {}
+
+type UpdateCommitteeGovAction struct {
+	// TODO
+}
+
+func (a UpdateCommitteeGovAction) isGovAction() {}
+
+type NewConstitutionGovAction struct {
+	// TODO
+}
+
+func (a NewConstitutionGovAction) isGovAction() {}
+
+type InfoGovAction struct {
+	// TODO
+}
+
+func (a InfoGovAction) isGovAction() {}
+
 type ConwayTransaction struct {
 	cbor.StructAsArray
 	cbor.DecodeStoreCbor